Description of Prior Art
It is well known that the performance of an internal combustion engine can be improved by varying the length of the exhaust system in response to changes in engine speed. Particularly important is the distance from the exhaust port to any divergent section, from which an exhaust pulse is reflected with a negative amplitude, and the distance from the exhaust port to any convergent section from which exhaust pulses are reflected with a positive amplitude.
Previous inventions have disclosed a variety of means for varying the length of exhaust systems and in particular for two-cycle engines where the exhaust system has a profound effect on engine performance. U.S. Pat. Nos. 2,102,559 (Kadenacy 1937) and 3,254,484 (Kopper 1966) disclose exhaust systems of variable length provided by axial extension of the exhaust pipes. This method has been utilized in a number of two-cycle applications but has the major disadvantage of requiring the displacement of the external structure of the exhaust system down stream of the axial extension section. This in turn makes it difficult to accommodate the exhaust system and provide rapid response to changes in engine speed. The Kadenacy patent (2,102,559) discloses a variation which includes the axial extension of an overlapping, co-axial section which, while reducing the displacement of the exhaust system, results in non-streamlined gas flow and poor exhaust pulse dynamics between the co-axial sections.
It is envisaged that the axial extension of the previously mentioned designs could be modified such that the extension remains along the center line of the extendable section but rather than linear follows an arced path. Such a modification would improve the physical operations of the axial extension design but would not overcome the other afore mentioned disadvantages.
A number of patents have disclosed means for varying the distance from an exhaust port to a reflective convergent section of an exhaust system. Examples of such patents include U.S. Pat. Nos. 3,703,937 (Tenney 1972), 3,726,092 (Raczuk 1973) and 4,941,319 (Yamamota 1990). In practice, while some performance improvements are provided this method does not provide optimal performance because it does not vary the important distance between the exhaust port and any section, such as a divergent section, upstream of the convergent section.
U.S. Pat. No. 5,134,850 (Saito 1992) discloses a means for varying the distance between an exhaust port and a point of divergence in the exhaust passage by way of a butterfly valve which controls the communication between merging exhaust passages. This method has the disadvantages of providing only a sudden point of divergence at the open butterfly valve, which does not efficiently utilize the energy of an exhaust pulse, and it does not influence the distance from the exhaust port to any point down stream of the divergence.